Idiot

The term idiot was formerly used to describe a person whose intelligence was too low to function without constant supervision. The term was not applied to toddlers or babies.

Higher up the intelligence scale were imbeciles, then morons. Imbeciles could be trusted for short periods without supervision, but could not lead independent lives. Morons could support themselves and live independently.

IQ scale:

idiot - 25 and below

imbecile - 25 to 50

moron - 50 to 80

Note that advocates have registered strong objections to the use of these terms and that IQ testers stopped using them in the late 20th century.
Individuals with such intellectual disabilities are now diagnosed with mild, moderate, or severe mental retardation.